Almost all vehicles have a parking brake mechanism, and most of these vehicles have a hand lever or foot activated lever for actuating the parking brake mechanism. Typically, these levers are attached to a cable that causes displacement of the cable and operation of the attached parking brake mechanism. There is a need to provide an electrically actuated cable mechanism that operates reliably with a minimum number of parts and has a low cost, so that manual lever actuated mechanisms may be replaced or easily selectively replaced.
It is known in the art to provide electric motor-driven brake systems. Examples of conventional technology in the motor-driven brake systems art are described in U.S. Pat. Nos. 4,795,002 to Burgei et al.; 4,865,165 to Taig; 5,590,744 to Belmond; 4,281,736 to Lizzio; 4,629,043 to Matsuo et al; 4,561,527 to Nakamoto et al; 5,180,038 to Arnold et al.; 5,485,764 to Sugimoto et al; and 5,542,513 to Reyes.
U.S. Pat. No. 5,485,764 to Sugimoto et al. describes a parking brake system for a vehicle that includes a unidirectional connecting member, such as a ratchet, that interconnects a brake operating member for manual operation of the parking brake and an automatic drive source for operating the parking brake. The device of Sugimoto suggests use of motor torque in brake application, but does not provide details by which use of motor torque can be economically measured.
U.S. Pat. No. 5,542,513 to Reyes describes an automatic parking brake that includes a second solenoid that activates the solenoid for the starter motor to engage a gear to activate the parking brake through the ignition switch. In addition, Reyes describes use of a mechanical tensioning limit device for controlling the tension of parking brake application. A problem with the use of tension limit switch in the cable, as described, is that this approach introduces unnecessary cost and complexity.
Prior art relating to setting travel using travel limit switches is ineffective, as the correct released state is not a fixed position, but a specified residual tension. It is also known to provide a mechanical tension limit feature for applying brakes, but no prior art exists in conjunction with these systems to control the released position of the brakes.
It is thus clear that conventional electrically powered parking brake systems lack effective controls for applying and releasing the brake and that these existing systems are relatively complex and costly. There is a need in this art for an electrically powered parking brake system that is reliable, self-adjusting in a relatively simple and reliable manner, cost-effective, uses a simple screw and drive tube assembly, and does not rely on additional mechanical activation, such as spring, or a complicated device or system for tension control and operation. There is a further need for a control system for an electrically powered parking brake system that produces a repeatable output within a relatively narrow range, in a manner sufficient to guarantee a level of braking, but not great enough to cause damage to associated components or require that the components be over-designed. There is a further need for a control system that relies upon a reliable and economical method and system for controlling the application of a parking brake, such as by use of motor current draw, which is proportional to output torque and sufficiently independent of other variables.